The present invention refers to a circuit for current injection control in analog switches, particularly in switches executed with MOSFET technology.
The MOSFET type analog switches are executed by means of two transistors, one N-Channel and the other P-Channel. The N-Channel transistor is controlled, by means of its own gate electrode, by a first control signal, while the P-Channel transistor is controlled, by means of its own gate electrode, by a second signal equal in amplitude to said first signal but in push-pull.
The analog switches are, for example, used in any N-Channel multiplexer/demultiplexer (or mux/dmux).
The field of application of such devices is, for example, the automobile sector.
For example in the U.S. Pat. No. 6,005,433 (Hale et al.) and in the U.S. Pat. No. 5,111,072 (Seidel et al.) techniques for controlling the charge injection during the turn off phase of an analog switch are described, which load was initially stored during the turn on phase of the switch itself. It therefore concerns a dynamic type of injection control.
In a mux/dmux in order to verify the quantity of current that flows from input to output when all the nxe2x88x921 switches are turned off and the nth switch is turned on, a particular control sequence is used, which provides for the injection of current in said turned off switches, a sequence which is also known by the name of xe2x80x9ccurrent injection controlxe2x80x9d.
In fact said sequence provides for a first phase wherein a current is forced on all the non-active Nxe2x88x921 channels (or switches), a second phase wherein the input of the only enabled channel is connected with a RS resistor and a third phase wherein the variation of the voltage on the common output channel is controlled.
The operation of current injection in the non-active switches enables said injected current passes through the conductive channel also turning on the switches that are turned off and causing a rise in the voltage present at the output.
In view of the state of the technique described, the object of the present invention is to construct a circuit which with an injected current is capable of preventing said current from passing through the conductive channel of the switch.
In accordance with the present invention, said object is reached by means of a circuit for current injection control comprising a first transistor having an input terminal, an output terminal and a control terminal, characterized in that it comprises a second transistor having an input terminal connected to said output terminal of said first transistor, an output terminal and a control terminal and comprising in addition coupling means placed between said input terminal and said control terminal of said second transistor, said coupling means being active when said first and second transistor are in the cut-off zone.
In addition, said object is also reached by means of a circuit for current injection control comprising a control signal suitable for controlling the operation of a first switch, consisting of a first transistor and a second transistor, the latter having its polarity opposite to said first transistor, characterized in that it comprises a second switch, consisting of a third and fourth transistor with polarity equal to said first and second transistor respectively, controlled by said control signal and connected to said first switch, so that the drain electrodes of said first switch are in common with the source electrodes of said second switch and also comprising a fifth and a sixth transistor polarized in conduction zone only when said control signal is low and having polarity equal to said first and second transistor respectively, controlled by said control signal so that the variation in output current is as low as possible.
Thanks to the present invention a circuit capable of limiting as much as possible the passage of current inside the switch can be constructed.
In addition thanks to the present inventive circuit it is also possible to prevent the parasitic bipolars of the MOSFET transistors composing the analog switch from turning on.